What causes the back scraping (Gua Sha) to turn black or purple?

Blackish-purple marks from back scraping (Gua Sha) indicate a severe presence of coldness in the body; the darker the color, the more serious the condition. Purple marks suggest eczema, purplish-red indicates severe damp-heat in the body, bluish-purple nodules point to cardiac coldness, and black marks reflect cold syndromes. Deep red indicates internal heat. Additionally, overly aggressive scraping can also cause blackish-purple skin discoloration. Since individuals vary in constitution and tolerance, avoid applying excessive pressure during the first session, as this may easily result in dark purple marks. In such cases, applying a warm towel to the affected area usually allows symptoms to resolve spontaneously within 5 to 7 days.